为什么一元减算子在这个表达式中存在问题:( - 2)1? [英] Why is the unary minus operator problematic in this expression: (- 2) 1?
问题描述
以下所有表达式的评估都不会有任何不妥:
(+ 2)1 - - 3
(* 2)1 - 2
(( - )2)1 - 1
(2-)1 - 1
(/ 2)1 - - 0.5
(2 /)1 - 2.0
但这不是: p>
( - 2)1 - 推断的类型不明确
GHC会抛出一些关于推断类型不明确的错误。为什么?
这些括号表达式中的每一个,但( - 2)
(编辑:和(( - )2)
)是部分,即函数采用一个参数,并且将它放在infix operator(见 haskell.org wiki )。
( - 2)
不是函数,而是数字(负数2): p>
λ> :t(-2)
(-2):: Num a => a
如果你写了
λ> (-2)1
看起来您正在尝试应用( -2)
(一个数字)为 1
(这是不可能的),GHCi正确地抱怨:
无法推论(Num(a0 - > t))
从上下文中的'it'
的歧义性检查(a - > t),Num a)
由'it'的推断类型限定:(Num(a-> t),Num a)=> t
at< interactive>:3:1-6
类型变量'a0'不明确
当检查'it'
具有推断类型'forall a t 。 (Num(a→t),Num(a)=> t'
可能的原因:推断的类型不明确
如果您想要一个减去您可以使用
(减去2)$ 2
b $ b
比较其类型,
λ> :t(减2)
(减2):: Num a => a - > a
到( - 2)
(见上文)。
术语附录(OP编辑后)
用括号括起minus运算符将其变成一个具有两个参数的普通(前缀)函数;因此
(( - )2)
不是一个部分,而是一个部分应用的函数。 All of the following expressions get evaluated without mishap:
(+2) 1 -- 3
(*2) 1 -- 2
((-)2) 1 -- 1
(2-) 1 -- 1
(/2) 1 -- 0.5
(2/) 1 -- 2.0
but not this one:
(-2) 1 -- the inferred type is ambiguous
GHC throws some error about the inferred type being ambiguous. Why?
Each of these parenthesized expressions but (-2)
(edit: and ((-) 2)
) are sections, i.e. functions that take one argument and "put it on the missing side of the infix operator" (see this haskell.org wiki).
(-2)
is not a function, but a number (negative 2):
λ> :t (-2)
(-2) :: Num a => a
If you write
λ> (-2) 1
it looks like you're trying to apply (-2)
(a number) to 1
(which is not possible), and GHCi rightfully complains:
Could not deduce (Num (a0 -> t))
arising from the ambiguity check for ‘it’
from the context (Num (a -> t), Num a)
bound by the inferred type for ‘it’: (Num (a -> t), Num a) => t
at <interactive>:3:1-6
The type variable ‘a0’ is ambiguous
When checking that ‘it’
has the inferred type ‘forall a t. (Num (a -> t), Num a) => t’
Probable cause: the inferred type is ambiguous
If you want a function that subtracts 2
from another number, you can use
(subtract 2)
Compare its type,
λ> :t (subtract 2)
(subtract 2) :: Num a => a -> a
to that of (-2)
(see above).
Terminology addendum (after OP's edit)
Parenthesizing the minus operator turns it into a normal (prefix) function that takes two arguments; therefore ((-) 2)
is not a section, but a partially applied function.
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